There is documentation from Sugar about how to make reports. This section of the documentation, in step 6, for example, mentions including the username field from the related Assigned to User related module.

There are also some instructions for building various types of reports HERE.

Using similar methodology, you can make an Accounts report and include fields from related Opportunities, or create an Opportunities report and use fields from related Accounts.

If you are running into an issue while attempting to implement this methodology described in this documentation, please follow up with details about what you have implemented as filters, display columns, etc., and how your results differ from your expectation.